Partial cone structure

The conformational isomers of a cahxarene with four phenol residues are shown in Fig. 2.17. The isomers vary in terms of the orientations of their phenol groups (a) has a cone structure with all of the phenols pointing to the same direction (b) has a partial cone structure with one phenol pointing in a different direction to the others (c) has a 1,3-alternate structure with neighboring phenols pointing in opposite directions. These isomeric hosts have different selectivities for metal ion inclusion in the upper cavity and the lower cavity. Of course, changing the number of phenol residues alters the guest size appropriate for effective inclusion. 

In supported liquid membranes (SLMs), the transport of potassium salts is always dependent on the difhision coefiScient of the complex through the membrane with the only exception for the derivative (12p), in the partial cone structure, wliere the rate limiting step is the release of the salt in the receiving phase. Both in transport (SLM) and in detection with ion selective field-effect transistors (ISFETs) the derivative in the 1,3-alternate structure (12a) shows higher selectivity than valinomycin. ... 

By changing the nature of the base and the solvent a certain control of the stereochemical outcome has been achieved in few alkylation [15] and acylation [24] reactions of calix[4]arenes. Recently, in collaboration with the group of Prof. D.N. Reinhoudt, several stereoisomeric calix[4]arene crown ethers have been synthesized and their ionophoric properties toward alkali metal cations determined by the Gam s extraction method [25]. In all cases it was found that these ligands are very selective for potassium cation and that the compounds in the partial cone structure are more efficient and more selective than those in the fixed cone conformation. The highest difference in... 

Alkylation of (l,2)-di[(2-pyridylmethyl)oxy]calix[4]arenes in THF in the presence of NaH produces the achiral his(syn-prvxinially) functionalized calix[4]arenes (cone conformers) in good yield [18]. On the other hand, when alkylation of 3a is conducted in DMF in the presence of CS2CO3, chiral partial cone structures are... 

Apart from NMR spectral patterns showing moleculeu asymmetry, evidence of chireility for cone 10 and partial cone structures 11 and 13 was provided by the addition of Pirkle s reagent (S)-(-H)-(9-anthryl)-2,2,2-trifluoroethanol to a chloroform solution of each calixarene, which caused doubling of (in principle) all signals. 

The Na+, K+, Rb+, and Cs+ complexes were simulated in the cone, partial-cone, and 1,3-alternate conformations, first without counterion. It was found that the smallest Na+ lies in a very deep position, surrounded by phenoxy-oxygens, not involving crown ether oxygens. As M+ gets bigger, it moves up to the crown. Picrate as the counterion remains an intimate pair with the complexed M+. The structure of the Na+ complex is different, compared with the host-M+ complex, because the picrate counterion pulls Na+ more exo to the crown region. On the contrary, for Cs+, the structures with and without picrate are very close, which demonstrates the good fit between Cs+ and the 1,3-alternate host. 

Figure 12 Molecular structure and stable conformers of p /ert-butycalix14 arcnc (a) cone (b) partial-cone (c) 1,3-alternate (d) 1,2-altemate.

The possible number of inherently chiral structures and conformers further increases if the calixarene contains both different phenolic units and different bridges in the macrocyclic skeleton. For example, two chiral monoethers 88a,b are available from dihomooxacalix[4]arenes (one -CH2-0-CH2- bridge instead of -CH2-).17188b is the preferred product of the mono-O-alkylation, since the negative charge of the respective phenoxide anion is better stabilized by intramolecular hydrogen bonds due to the smaller distance between the phenoxide anion and the hydroxy groups. Tetraketone derivatives (Y = CH,-C(0)-R) in the two possible partial cone conformations, have been prepared in moderate yields. 

The structures and abbreviations used for designating the calix[4]arenes are shown in (5). These calix[4]arenes, have both a wide (upper) and a narrow (lower) rim that can be chemically modified to produce complexants that are selective for particular metal ions. In the simple calixarene framework the wide rim has hydrocarbon functionalities, and the narrow rim phenolic groups. Calixarenes are conformationally mobile, and the extreme structures for the calix[4]arenes have been termed the cone, partial cone, 1,3-alternate, and 1,2-alternate conformations (6). Because of the conical geometry of the calix[4]arene structure, the cavity size of the wide rim is larger than that of the narrow rim. 

Relatively few X-ray structures of calix[4]arenes in the partial cone conformation have been reported (see ref. 1, pp. 106-107), most of them involving tetra-0-... 

Figure 4.9 X-Ray crystallographic structures of a) partial cone conformer 91d (Taken from Harrowfield et (b), 3-alternate conformer 92b...

The X-ray structure of calix[4]arene tetraquinone " shows it to be in a flattened partial cone conformer. Similarly, the dialkyl ether diquinones 96a, 96b, and 96c all assume the partial cone conformation but differ with respect to the orientation of the four residues in the cyclic array in 96a and 96b the alkoxyaryl... 

Figure 6.2 X-Ray crystallographic structures of complexes of calixcrowns (a) 270b in cone conformation and (b) 270c in partial cone conformation (Taken from Ugozzoli et al. )...

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